For a long time, with JIT enabled builds I am facing with asserts when running either jsc or QtLauncher inside valgrind.
Test cases are simple:
$ valgrind WebKitBuild/Debug/bin/QtLauncher #and browsing a little
$ valgrind WebkitBuild/Debug/JavaScriptCore/jsc SunSpider/tests/sunspider-0.9.1/3d-cube.js
Both of them asserts here:
ASSERTION FAILED: !callLinkInfo->isLinked()
(../../../JavaScriptCore/jit/JIT.cpp:610 static void JSC::JIT::linkCall(JSC::JSFunction*, JSC::CodeBlock*, JSC::CodeBlock*, JSC::JITCode&, JSC::CallLinkInfo*, int, JSC::JSGlobalData*))
As I remember, in the early days of the JIT there were no such problems.
Of course, we can not be sure that the bug is not in valgrind.
Personally I think that it would be useful to clarify the problem.

I have found a solution: use "--smc-check=all".
This is needed because the JIT::linkCall function (and some other JIT staff) modifying the jitted code, so valgrind must check self modification.
Alternatively we could use valgrind's client request mechanism. That would be cheaper, but I am dubious about how the apple folks would feel about to introduce valgrind into the codebase (we should have at least a build option, and a define for valgrind).